1. Field of the Invention
The present invention relates to a method of controlling an air/fuel ratio of an internal combustion engine mounted on a vehicle.
2. Description Of Background Information
Various air/fuel ratio control systems for internal combustion engines are known from, for example, Japanese Patent Publication No. 55-3533, which systems regulate the air/fuel ratio of mixture to be supplied to the engine toward a target air/fuel ratio value by a feedback control operation in which a control parameter for the air/fuel ratio control is set in response to the output signal of an oxygen concentration sensor disposed at the exhaust system of the engines thereby to regulate the volume of air or fuel the mixture to be supplied to the engines. The air/fuel ratio control parameter may be, for example, a valve-open period in an intake side secondary air supply system, or a fuel injection period in a fuel injection system.
Such air/fuel ratio control systems generally perform a control operation in which the feedback control of the air/fuel ratio according to the output signal of the oxygen concentration sensor is stopped and the air/fuel ratio is controlled to be leaner than the target air/fuel ratio during the engine is idling, so as to improve the fuel economy. However, as illustrated in FIG. 1, the fluctuation of the engine speed under the idling condition becomes large as the air/fuel ratio becomes lean. (In FIG. 1, .DELTA.Ne represents the width of the flucturation of the engine speed.) Thus, leaning of the air/fuel ratio not merely improves the fuel economy but also causes to deteriorate the driveability and to increase the vibration of the vehicle body.
On the other hand, in the case of internal combustion engines mounted on a vehicle, there is provided an idle rotational speed control device by which the throttle valve is forcibly opened so that the power current is stably supplied during an idling condition in which an electric unit having a relatively large electric load such as a head light and an air conditioner is in operation. When the throttle valve is opened by the idle rotational speed control device, the rotational speed of the engine is raised to increase the electric power generated by the generator. On the other hand, when the throttle valve is opened by the idle rotational speed control device, the weight of the intake air "Gair" increases as well. As shown in FIG. 2, the width .DELTA.Ne of the fluctuation of the rotational speed of the engine decreases as the weight of the intake air Gair increases. Thus, the driveability of the vehicle is improved and the vibration of the vehicle body is decreased as well. However, the opening of the throttle valve also results in an increase of the fuel consumption. Therefore, although the electric power supply to the unit of large electric load is assured and the driveability of the vehicle is improved, it was undesirable that the operation of the idle rotational speed control device causes an increase of the fuel consumption.